This invention relates to the preparation of silicone foams, and more particularly to a method for their preparation which is characterized by improved safety and product quality.
As used herein, the term "silicone foam" designates a polyorganosiloxane composition in the form of a foam. Silicone foams are well known in the art and their preparation is described in a number of patents. A method of high current interest involves contact between a polyorganosiloxane having vinyl groups attached to silicon, a polysiloxane having hydrogen atoms attached to silicon and a hydroxy compound. Upon contact with a platinum group catalyst, two reactions occur: hydrosilylation of the vinyl groups by interaction with the silicon-hydrogen moieties, and generation of hydrogen by interaction of the hydroxy compound with the silicon-hydrogen moieties. The hydrogen thus generated acts as a blowing agent to produce the foam.
Numerous types of hydroxy compounds have been employed in this method of preparing foam. They include water and silanols (i.e., compounds containing at least one Si--OH group), which, however, tend to afford foams having densities too high for many applications.
Also employed, as disclosed, for example, in U.S. Pat. No. 4,026,843, are low molecular weight alkanols such as methanol and ethanol. Methanol, in particular, is known to produce a low density foam with small cells. However, its very low vapor pressure and low boiling point dictate that it be added just prior to foaming, rather than incorporated in a package with a useful shelf life. In addition, its low flash point (11.degree. C.) causes safety concerns.
The replacement of low molecular weight alkanols with higher molecular weight materials has been suggested. For example, U.S. Pat. No. 4,590,222 discloses the use of alkanols and alkanediols having 3-12 carbon atoms, with lauryl alcohol being singled out. The use of diols of various carbon chain lengths is also disclosed in U.S. Pat. No. 4,871,781.
Experimentation has shown that alkanols having 3 or 4 carbon atoms also afford low density foams; however, they are only marginally superior to methanol and ethanol in vapor pressure and safety aspects. The use of higher molecular weight alkanols produces foamed materials having very large cells, typically with a diameter greater than 1 mm., rather than the desired foams of small cell size. Polyols, including diols such as 1,6-hexanediol, produce foams with small cell size but relatively high density, typically greater than 0.22 g/cm.sup.3.
Thus, it remains of interest to produce silicone foams having small mean cell size, typically up to 0.5 mm., and low density, typically up to 0.21 g/cm.sup.3. The present invention provides a method for preparing such foams and compositions useful in said method.